In digital cameras that have been rapidly becoming popular in recent years, three-group lenses are used for achieving miniaturization and improving aberration correction. Frequently used are rear-focus type three-group zoom lenses that provide closer focusing by moving the third lens group from the object side toward the image side. Japanese Laid-Open Patent Application H10-133115, Japanese Laid-Open Patent Application 2001-296476, and Japanese Laid-Open Patent Application 2003-035868 are exemplary publications that disclose such three-group zoom lenses.
The three-group zoom lens described in Japanese Laid-Open Patent Application H10-133115 includes, in order from the object side, a first lens group having negative refractive power, a second lens group having positive refractive power and an aperture stop, and a third lens group having positive refractive power. During zooming from the wide-angle end toward the telephoto end, the first lens group moves first toward the image side and then toward the object side so that the locus of points of the position of the first lens group along the optical axis between the wide-angle end and the telephoto end, as viewed in cross-section including the optical axis, follows an arc that is convex toward the image side, the second lens group moves monotonically toward the object side, and the third lens group moves first toward the object side and then toward the image side so that the locus of points of the position of the third lens group along the optical axis between the wide-angle end and the telephoto end, as viewed in cross-section including the optical axis, follows an arc that is convex toward the object side.
The three-group zoom lens described in Japanese Laid-Open Patent Application 2001-296476 includes, in order from the object side, a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group having positive refractive power. During zooming from the wide-angle end toward the telephoto end, the third lens group moves monotonically toward the image side or moves in an arc that is convex toward the image side by first moving toward the image side and then moving away from the image side.
Although conventional zoom lenses have been designed in order to achieve greater miniaturization, a larger zoom ratio, and a higher resolution, in recent years even greater miniaturization, larger zoom ratio, and higher resolution have been desired.
Also, in the three-group zoom lens described in Japanese Laid-Open Patent Application H10-133115, the absolute value of the ratio of the focal length of the first lens group divided by the focal length of the entire zoom lens is about 2.4 to 2.6. Although it can satisfy the demands of miniaturization, zoom ratio, and resolution up to a zoom ratio of about three, when the zoom ratio exceeds three, the total length of the zoom lens increases so much that it does not satisfy the demands for miniaturization.
Additionally, in the three-group zoom lens described in Japanese Laid-Open Patent Application 2001-296476, because the third lens group moves monotonically toward the image side or moves in an arc that is convex toward the image side as discussed above, although it can satisfy the demands of miniaturization, high zoom ratio and higher resolution up to a zoom ratio of about three, when the zoom ratio exceeds three, correction of curvature of field at intermediate focal lengths becomes difficult.